Casing collar locator tools are instruments used in logging operations in boreholes that are cased with metal pipe (typically ferromagnetic steel) to provide structural support for the borehole, to ensure that subsurface fluids such as water, oil and gas do not enter the wellbore from undesired formations or escape into underground rock formations. It is very important in such logging operations to be able to ascertain the exact depth of various tools that are used in production operations, and a collar locator is typically used to count the total number of lengths of casing traversed in the descent down the borehole by a string of tools from the wellhead (located at ground level) to a desired depth in the wellbore.
Prior art casing collar locator tools are used to determine the locations of the joints between sections of steel casing, where a steel "collar", or threaded pipe typically of larger diameter than the casing, is screwed to the adjacent ends of both pieces of casing being connected. A standard prior art collar locator tool uses the principal of Faraday induction, employing a strong magnet to generate a magnetic field and a coil in which a voltage is induced due to the motion of the coil through the magnetic field perturbation caused by the magnetic discontinuity created by the gap between two sections of casing. These prior art "collar locators" are actually casing joint locators - they detect the gap between sections of casing, not the presence of a collar.
The primary problems with these prior art tools are: 1) they are typically speed-dependent, in that the amplitude of the received signal is proportional to the speed at which the tool is being pulled through the bore hole. It is often desirable, however, to log relatively slowly in order to get good depth resolution. Slow logging may also be desirable for other sensors in the tool-string as well, but it makes determining the location of collars more difficult when permanent magnet tools are used. 2) "Flush-joint" collars, which are specially designed casing joints that reduce the casing gap essentially to zero, and which have an inside diameter that matches that of the casing pipe, are difficult to detect using typical prior art casing collar locators because there is no "gap" between sections of casing pipe. 3) It is often desirable to centralize the tool string in the casing, because many types of tools that may be used with a collar locator must be centralized. However, many prior art collar locators must be decentralized (pressed against the wall of the casing) to work properly, or they provide the worst signal-to-noise ratio if they are centralized due to the large gap separating the locator tool from the casing wall. 4) Typical prior art casing collar locators include powerful permanent magnets to induce magnetic fields in the casing. These permanent magnets often suffer demagnetization when exposed to harsh conditions in a bore hole, and they also present a handling and safety hazard due to their tendency to pick up magnetic "trash."
Many attempts have been made to try to resolve these problems with prior art collar locators, such as adding sensitive electronic amplifiers to increase the signal level, and using multiple coils disposed to improve depth resolution, but none of the known prior art methods are able to solve all of the problems simultaneously. Accurate and reliable detection of the location of flush joint collars remains an unsolved problem in the industry that is not addressed by prior art collar locators.